I would like more information about these "Security updates" that we are being continually expected to download. and update. What exactly are they and what are you putting into our computers. I have just recently had two computers crash after updat

https://support.apple.com/HT201222

The link I provided should reveal a lot of details for various security update releases which is about as far as Apple will go telling you what each one has. I don't really know that those will help you much with what you which to know. Nor do I suspect all those problems are due to security updates clashing with your systems. A "slow Mac" post here can often be traced to other software being present such as a virus scanner, or a cleaning tool, or maybe a defective drive.

https://support.apple.com/kb/index?page=search&q=security%20update%20mojave&product=&doctype=&currentPage=1&includeArchived=false&locale=en_US

Any update will change the system a little (or a lot) and, depending on other software, it may present compatibility issues. Other than that, I would certainly trust the manufacturer of my computer to not issue anything that is not safe for my computer.

As to what Apple is "putting in these updates" - Apple would certainly NOT publish details on security installs - that would not be advisable.